1. Field of the Invention.
The present invention relates generally to power supplies and, more particularly, to such a power supply for use with pulse width modulated amplifiers.
2. Description of the Related Art.
Large amplifiers made for magnetic resonance imaging (MRI) gradient amplifiers have often comprised a major portion of the cost of the overall MRI system. As gradient amplifier sizes continue to increase to provide increased functionality for imaging, the size and cost of the amplifier's DC power supplies has likewise risen.
Pulse Width Modulated (PWM) amplifiers offer a very attractive simplification to the DC power supply system in that they typically can share one supply between all three axes of amplifiers, X, Y and Z. Since all three axes are not driven to full capacity at any one time, the aggregate DC supply demand is not three times that of the maximum demand of any one axis. A worst case would be that the total demand is only twice the maximum demand of one axis.
If three separate supplies had been required, the result would be that the system would need to make three supplies of the worst case size to meet any possible demand. Thus it is evident that by sharing supplies the PWM amplifier has reduced the power supply to 2/3 of the size of designs requiring three separate and isolated supplies. Since the supply is both smaller and simpler, it would be typical that the power supply cost has been halved by supply sharing.
The reason that PWM amplifiers can operate with a shared supply is that they do not require topologies such as grounded bridges which have floating supplies to overcome voltage based limitations of their output power controlling semiconductor devices. When semiconductors are used in non-dissipative modes as switches, they are less limited by voltage induced failures and higher voltages are possible.
The opportunities for optimization do not end with the improvements due to commonality. A further benefit derives when it is noted that the construction of very large amplifiers no longer require the use of galvanic isolation to isolate the output DC potentials from the AC mains potentials. Traditionally, isolation is done in small products to minimize the lethal exposure of users to primary-side power when contacting the secondary side. With a large amplifier having DC outputs of hundreds of volts and hundreds of amps, any direct contact of a user with the secondary side would be fully lethal, whether there were galvanic isolation or not. Safety must be derived by other means.
Strangely, gradient amplifiers have retained the isolated designs that were appropriate for small products in other applications. Galvanic isolation provides no useful feature once AC mains line transients have been filtered and arrested. By continuing to design with galvanic isolation, the cost and size of the supply have been inflated.
Ideally, a gradient amplifier would be operated with two DC supply feeds which are electrically centered (plus and minus voltage) about ground. This allows the amplifier to operate with a no output of zero volts on all of its full-bridge output terminals. Failure to so operate can result in both a hazard to amplifier technicians exposed to net DC voltages on the load when there is no signal and electrolysis within water cooled gradient coils that allow direct impingement of the cooling water with the gradient coils.
Typically, non-galvanically isolated supplies are less expensive to implement than those requiring all power to flow through transformers having isolated primaries and secondaries. Large amounts of unregulated power can be obtained in a non-isolated manner by simply rectifying the AC mains (three-phase). Being three-phase the resultant ripple voltage on the DC output is relatively small compared to the DC component.
If the three-phase input AC power does not provide connection to the neutral feed, all non-isolated full-wave rectifier circuits will of necessity be of classic 6-pulse delta form. If the neutral is provided, the rectifier could also be of 6-pulse wye form with the neutral at DC common potential.
PWM gradient amplifiers do not require power supplies of varying voltages. Ideally, the operating DC voltages are fixed and not subject to variations in line or load. Traditionally, the design of PWM gradient amplifier DC power supplies has in cluded regulating power supplies that would be capable of regulating output voltage over a wide range of voltage. Such regulators are appropriate for laboratory use where the desired voltage can vary from use to use. The only gradient amplifier requirement for diversity of output voltage is to be able to shut-down (zero output volts) when required by fault or safety related conditions .